(1) Field of the Invention
The present invention generally relates to methods of producing materials with nanocrystalline microstructures, and more particularly to producing such materials by machining and then using the nanocrystalline material to form a product.
(2) Description of the Related Art
Significant benefits can be gained by deforming metal alloys through the application of very large plastic strains. Principal among these are microstructure refinement and enhanced mechanical and physical properties. Of particular current interest is the use of xe2x80x9cseverexe2x80x9d plastic deformation (SPD) to produce bulk solids with ultra-fine grained microstructures (UFGS), especially nanocrystalline structures (NS) characterized by their atoms arranged in crystals with a nominal dimension of less than one micrometer. Nanocrystalline solids have become of interest because they appear to have significant ductility, formability and resistance to crack propagation, and possess interesting chemical, optical, magnetic and electrical properties. Nanocrystalline solids also appear to respond to radiation and mechanical stress quite differently than microcrystalline materials (crystals with a nominal dimension of one micrometer to less than one millimeter), and their response can be varied by changing the crystal size. Materials made by consolidating nanocrystalline powders have also been shown to have enhanced attributes not typically found in conventional materials. As a result, nanocrystalline materials are believed to have significant potential for use in industrial applications, provided they can be manufactured in a cost-effective manner.
Multi-stage deformation processing is one of the most widely used experimental approaches to studying microstructural changes produced by very large strain deformation. Notable examples include such techniques as rolling, drawing and equal channel angular extrusion (ECAE). In this approach, very large plastic strains (true plastic strains of four or more) are imposed in a specimen by the cumulative application of deformation in multiple stages, the effective strain in each stage of deformation being on the order of one. The formation of micro- and nanocrystalline structures has been demonstrated in a variety of ductile metals and alloys using multi-stage deformation processing. However, there are significant limitations and disadvantages with this processing technique. A significant limitation is the inability to induce large strains in very strong materials, such as tool steels. Other limitations include the inability to impose a strain of much greater than one in a single stage of deformation, the considerable uncertainty of the deformation field, and the minimal control over the important variables of the deformation fieldxe2x80x94such as strain, temperature, strain rate and phase transformationsxe2x80x94that are expected to have a major influence on the evolution of microstructure and material properties.
The most widely used technique for synthesizing nanocrystalline metals has been by condensation of metal atoms from the vapor phase. In this technique, the metal is evaporated by heating and the evaporated atoms then cooled by exposure to an inert gas such as helium or argon to prevent chemical reactions, thereby enabling the purity of the metal to be maintained. The cooled atoms condense into single-crystal clusters with sizes typically in the range of 1 to 200 nm. The production of ceramic nanocrystals is similar, except that evaporated metal atoms are made to react with an appropriate gas, e.g., oxygen in the case of oxide ceramics, before they are allowed to condense. The resulting crystals may be compacted and sintered to form an article, often at a sintering temperature lower than that required for a microcrystalline powder of the same material. While suitable for making powders and small compacted samples with excellent control over particle size, the condensation method is at present not practical for most applications other than experimental. A particularly limiting aspect of the condensation method is the inability to form nanocrystalline materials of alloys because of the difficulty of controlling the composition of the material from the vapor phase. Another limiting aspect of the condensation method is that high green densities are much harder to achieve as a result of the nano-size particles produced. Other methods that have been explored to synthesize nanocrystals include aerosol, sol-gel, high-energy ball-milling, and hydrothermal processes. However, these techniques cannot produce nanocrystalline materials at a cost acceptable for practical applications.
From the above, it can be seen that it would be desirable if a more controllable and preferably low-cost approach were available for synthesizing nanocrystalline solids for use in the manufacture of products. It would be particularly desirably if such an approach were capable of producing nanocrystalline solids of a wide variety of materials, including very hard materials and alloys that are difficult or impossible to process using prior art techniques.
The present invention provides a product in which at least a portion of the product has a nanocrystalline microstructure, and a method of forming the product. The method generally entails machining a body in a manner that produces chips consisting entirely of nano-crystals as a result of the machining operation being performed in a manner that imposes a sufficiently large strain deformation. The body can be formed of a variety of materials, including metals, metal alloys, and ceramic materials. Furthermore, the body may have a microstructure that is essentially free of nano-crystals, and may even have a single-crystal microstructure. The chips produced by the machining operation may be in the form of particulates, ribbons, wires, filaments and/or platelets. The chips are then used to form the product. According to one aspect of the invention, the chips are consolidated (with or without comminution) to form the product, such that the product is essentially a monolithic material consisting essentially or entirely of nano-crystals, or of grains grown from nano-crystals. According to another aspect of the invention, the chips are dispersed in a matrix material, such that the product is a composite material in which the chips are dispersed as a reinforcement material.
The above features of the invention are based on the determination that nanocrystalline structures can be formed in materials by machining under appropriate conditions to produce very large strain deformation, including high strain rates, such as a plastic strain of about 0.5 to about 10 and a strain rate of up to 106 per second. Machining processes believed to be capable of producing suitable nanocrystalline structures include cutting and abrasion techniques. Cutting speed does not appear to be determinative, such that essentially any cutting speed can be used if a cutting tool is used to perform the machining operation. Because the production method for the chips is a machining operation whose parameters can be precisely controlled, the desired nanocrystalline microstructure for the chips can be accurately and repeatably obtained for a given body material. Furthermore, the machining operation can be adjusted to produce chips of various grain sizes and macroscopic shapes for use in a variety of application. The production of nanocrystalline chips can often be achieved with this invention without having any negative impact on the article being machined, such that nanocrystalline chips can be produced as a useful byproduct of an existing manufacturing operation. If the byproduct of such a manufacturing operation, it is notable that the chips produced and utilized by this invention would previously have been viewed as scrap produced by the operation, and therefore simply discarded or melted for recycling.
In view of the above, the present invention provides a controllable and low-cost method for synthesizing nanocrystalline solids that can be used to produce monolithic and composite products. The method of this invention also makes possible the capability of producing nanocrystalline solids from materials that have been difficult or impossible to process using prior art techniques, such as very hard materials that cannot be processed by multi-stage deformation processes, and alloys that cannot be processed by the condensation method.
Other objects and advantages of this invention will be better appreciated from the following detailed description.